It is well known in the art that various relatively rigid and/or brittle polymers of monovinylidene aromatic monomers can be made more impact resistant by the inclusion of amounts of various types of elastomeric materials (rubbers) into a matrix or continuous phase of said brittle polymer. Usually the elastomeric materials are in the form of discrete particles, such particles having amounts of the matrix polymer or a polymer similar thereto graft polymerized to particles. These types of rubber-modified, impact-resistant polymeric compositions are commonly known and referred to as graft copolymers or polyblends. Among the best known of these types of compositions are high impact polystyrene (HIPS), poly(styrene-acrylonitrile) reinforced with a butadiene-containing elastomer (ABS), and compositions similar to these.
It is also well known in the art that the physical properties of these types of compositions are greatly affected by the relative amounts of elastomer particles having different sizes and structures. Larger rubber particles having diameters greater than about 0.5 micron contribute greatly to impact resistance, but tend to reduce the gloss of articles formed or molded from compositions containing them. On the other hand, when smaller modifying-rubber particles are used in polymer compositions, articles formed therefrom tend to be glossier, but are less impact resistant than if the same amount of larger-particle rubber was used.
Furthermore, concerning the structures of the individual rubber particles (i.e., rubber particle morphology), there are well-known advantages and disadvantages accompanying the use of either of the two main rubber particle types in rubber-modified polymer compositions. It is generally believed that the grafted rubber particles containing occlusions of matrix polymers therein, one of the two main particle types, provide more impact resistance than the same amount of rubber in the form of similarly grafted solid rubber particles, the other main particle type. Such grafted, occlusion-containing rubber particles are usually formed and grafted in a mass-type or mass-suspension-type polymerization process where a previously-produced rubber is dissolved in a solution comprising one or more polymerizable monomer(s), which monomers are thereafter polymerized. Occlusion-containing particles produced in such mass or mass-suspension types of processes or variations of these processes are hereinafter referred to as "mass particles".
It is also well known in the art that mass particles present in rubber-modified polymeric compositions can have a very detrimental effect on the gloss of articles formed therefrom. In spite of the disadvantages of mass particles, however, they are a very desirable constituent of rubber-modified polymer compositions. One basis for their desirability is that, probably due to their occluded structure, they provide a great deal of impact resistance for the amount of rubber which is actually present. Other desirable facets of including mass particles in rubber-modified polymer compositions include the ability to utilize a wide variety of rubber compositions and types and the economy and efficiency of the mass-type processes by which they are formed.
Solid or non-occluded rubber particles, the other main type of rubber particle, are usually achieved via emulsion polymerization of the rubber in aqueous latex. After the rubber is made, polymerizable, graftable monomers (e.g., styrene and/or acrylonitrile) are usually added to the rubber-containing latex and polymerized to form the graft portion as well as amounts of matrix polymer. These non-occluded type rubber particles, produced via emulsion polymerization are hereinafter referred to as "emulsion particles". Under most circumstances, however, emulsion polymerization techniques are generally economically feasible for the production of rubber particles having diameters less than about 0.25 microns or so.
As is also generally known in the art, there are many individual characteristics of rubber particles, given a desired size and particle structure, which can be conveniently and separately controlled to optimize certain properties of the rubber-modified polymer compositions to which they are added. Some parameters which are subject to quite wide variation to affect the physical properties of the resultant compositions include the molecular weight of the mass rubber, the degree to which either mass or emulsion rubber is cross-linked, the amounts and types of different polymers which are grafted to the particles. The use of higher molecular weight rubbers in the mass particles produce rubber-modified polymer compositions which are tougher but have poorer gloss. In general, the physical properties of rubber-modified polymer compositions are optimized at intermediate crosslink densities and graft-to-substrate ratios, the extremes in either of these properties affecting most properties of the resultant compositions undesirably. Since the above-described rubber characteristics are easily controlled in the mass rubber particles, this is another incentive for their use.
As is obvious from the above discussion, and well known in the art, emulsion polymerization techniques are well-suited for preparation of small rubber particles, while mass-type processes or agglomeration of smaller emulsion particles can be used to achieve large particle sizes.
In view of these phenomena observed in production of rubber-modified polymer compositions, a great deal of effort has gone into achieving optimized physical properties by tailoring the rubber particle distribution (i.e., sizes and types of rubber particles and the amounts of different size and/or types rubber particles) in polymeric compositions. See, for example, U.S. Pat. Nos. 3,509,237; 3,576,910; 3,652,721; 3,663,656; 3,825,621; 3,903,199; 3,903,200; 3,928,494; 3,928,495; 3,931,356; 4,009,226; 4,009,227; 4,017,559; 4,224,414; 4,233,409; 4,250,271; 4,277,574, wherein various "bimodal" rubber particle distributions in ABS polymer compositions are disclosed.
The teachings of these patents can be broadly characterized as teaching that gloss and impact resistance can be affected by the sizes and types of rubber particles included in rubber-modified polymer compositions. Most of them teach that a substantial percentage of the particulate rubber must be of the small, emulsion-produced particle type to yield satisfactory, glossy, impact-resistant polymer compositions.